Efficient control of membrane fouling in MF by removal of biopolymers: Comparison of various pretreatments

Enhanced permeation performance of biofiltration-facilitated gravity-driven membrane (GDM) systems by in-situ application of UV and VUV: Comprehensive insights from thermodynamic and multi-omics perspectives

Biofouling is a major challenge limiting the practical application of biofiltration-facilitated gravity-driven membrane (GDM) systems in drinking water treatment. In this study, ultraviolet irradiation, including ultraviolet (UV) and vacuum ultraviolet (VUV) irradiation, was used for in-situ purification of membrane tanks to control membrane biofouling. After using UV and VUV, the permeate flux increased significantly by 26.1% and 78.3%, respectively, which was mainly due to the decreased cake layer resistance (Rc). The permeability of the biofouling layer improved after UV and VUV application, as evidenced by the increased surface porosity and decreased thickness. The contents of loosely bound extracellular proteins (LB-PN) and tightly bound extracellular proteins (TB-PN) in the biofouling layer were reduced after UV and VUV irradiation. The decreased LB-PN and TB-PN improved the interfacial free energy between the fouling itself and between the fouling and the membrane, which contributed to the reduction of interfacial cohesion and adhesion, resulting in a looser and thinner biofouling layer and a cleaner membrane. The concentration of protein-like material in the membrane tank decreased after UV and VUV irradiation, significantly altering the bacterial community structure on the membrane surface (Mantel's r > 0.7, p < 0.05). The changes in the metabolic state were responsible for the differences in the LB-PN and TB-PN contents. The inhibition of "Alanine, aspartate and glutamate metabolism" and "Glycine, serine and threonine metabolism" reduced amino acid biosynthesis, which restricted the secretion of LB-PN and TB-PN. Critical genera in the Proteobacteria phylum, such as Hirschia, Rhodobacter, Nordella, Candidatus_Berkiella, and Limnohabitans, were involved in metabolite transformation. Overall, the in-situ application of UV and VUV can be an effective alternative strategy to mitigate membrane biofouling, which would facilitate the practical application of biofiltration-facilitated GDM systems in drinking water treatment.

Effect of peroxydisulfate oxidation catalyzed with ordered mesoporous carbons on controlling ultrafiltration membrane fouling by algal organic matter

As typical ordered mesoporous carbons (OMCs) materials, CMK-3 and CMK-8 were proposed for catalyzing peroxydisulfate (PDS), and the OMCs/PDS process was combined with membrane filtration to remove algal extracellular organic matter and mitigate membrane fouling. The CMK-3/PDS process achieved substantial reduction of dissolved organic carbon and UV₂₅₄, followed by CMK-8/PDS. The degradation behavior of fluorescent organics demonstrated the superior performance of OMCs/PDS, while the decomposition of high molecular weight (MW) compounds and generation of lower MW organics were observed. Generally, CMK-3 possessed higher catalytic activity on PDS compared with CMK-8 and powdered activated carbon. The CMK-3/PDS process distinctly decreased the fouling resistances for polyether sulfone and polyvinylidene fluoride membranes, with the reversible resistance reduced by 59.5-83.2% and irreversible resistance declined by 71.7-73.0%. In the meanwhile, CMK-3/PDS prolonged the volumes to the transition period, and postponed the cake layer's generation. The characterization of the membrane morphologies and chemical compositions also showed effective alleviation of fouling. The generated SO₄^-, OH, O₂^- and ¹O₂ as major active oxidation species provided radical as well as non-radical reaction ways for pollutants removal. Overall, our study provides some new ideas for membrane-based combined water purification processes.

Online colloidal particle monitoring for controlled coagulation pretreatment to lower microfiltration membrane fouling at a potable water reuse facility

Fouling of microfiltration (MF)) membranes during water/wastewater treatment is predominantly caused by colloidal particles (size <1 µm) in the feed water. Until recently no online technology was available to directly measure the occurrence of colloidal particles in these waters. This study evaluated the viability of a novel online light scattering technology (Nanoparticle Tracking Analysis) to continuously monitor colloidal particles in the membrane feed water (a secondary-treated wastewater) for targeted removal by injecting coagulant at a dosage proportional to the measured concentration of colloidal particles. A diurnal variation was observed in the colloidal particle concentration in the feed water with the lowest concentration occurring at approximately 6 am and the highest concentration occurring after mid-day. The peak colloidal particle concentrations were 4 to 6 times higher than the lowest concentrations measured on the same day. Bench-scale studies were performed to develop a relationship between colloidal particle concentration and the optimum coagulant dosage required for their removal. Subsequently, a pilot-scale study was performed using two MF pilot units operated in parallel, one receiving targeted coagulant dosing and the other with no coagulant dosing, to demonstrate the effectiveness of targeted coagulant dosing in preventing membrane fouling. The pilot unit that received targeted coagulant dose experienced only 4 to 20% of the transmembrane pressure increase of the increase experienced by the pilot unit that received no coagulant. Evaluation of fouling resistance indicated that targeted coagulation improved flux by predominantly lowering the irreversible fouling. The filtrate water quality measured by colloidal particle concentration, chemical oxygen demand (COD), and turbidity were very similar for the two pilot units. This suggests that although the efficiency of particle and organic materials removal does not change with coagulant addition, the particles filtered by the membrane in the control unit contributed to membrane irreversible fouling, while in the coagulant-treated unit, the coagulated colloidal particles were removed away from the membrane into the backwash stream during the frequent backwash/air scour procedures.

Modeling the antifouling properties of atomic layer deposition surface-modified ceramic nanofiltration membranes

This work investigates the enhancement of antifouling properties of ceramic nanofiltration membranes by surface modification via atomic layer deposition (ALD) of TiO₂. Feed solutions containing bovine serum albumin (BSA), humic acid (HA) and sodium alginate (SA) were used as model foulants. The classic fouling mechanism models and the modified fouling indices (MFI) were deduced from the flux decline profiles. Surface roughness values of the ALD coated and uncoated membranes were 63 and 71 nm, respectively, while the contact angles were 34.2 and 59.5°, respectively. Thus, coating increased the water affinity of the membrane surfaces and consequently improved the anti-fouling properties. The MFI values and the classic fouling mechanism correlation coefficients for cake filtration for the ALD coated and the uncoated membrane upon SA fouling were 42,963 (R² = 0.82) and 143,365 sL^-2 (R² = 0.98), respectively, whereas the correlation coefficients for the combined foulants (SA + BSA + HA) were 267,185 (R² = 0.99) and 9569 sL^-2 (R² = 0.37), respectively. The study showed that ALD can effectively enhance the antifouling properties of ceramic membranes.

Targeting membrane fouling with low dose oxidant in drinking water treatment: Beneficial effect and biological mechanism

Membrane fouling is the principal factor that currently limits the performance of gravity-driven membrane (GDM) filtration systems in drinking water treatment. In this study, the benefits of applying a low dose (approximately 0.1 mg·L-1) of environmentally benign oxidants, both H2O2 and KMnO4, as a pretreatment to GDM filtration system has been evaluated in terms of reduced membrane fouling and treated water quality. While both oxidants improved permeate flux, the effect of KMnO4 was greater than H2O2. Both oxidants reduced the size of influent organic substances and those of large molecular weight (>20 kDa), such as biopolymers, disappeared. The thickness of the fouling layers was substantially reduced after oxidation, and the KMnO4 system had a markedly different physical structure of fouling layer, with an apparent sub-layer of δ-MnO2 nanosheets below a fouling sub-layer. The formation of the δ-MnO2 nanosheets sub-layer appeared to protect the underlying membrane pores from contamination by influent organics. Oxidation pretreatment reduced the presence of proteins and polysaccharides in the fouling layers and significantly altered the bacterial community structures (p < 0.01) and decreased biodiversity. The microbial species that secreted amounts of extracellular polymeric substances (EPS), such as Xanthobacter, were not eliminated in the H2O2 fouling layer, while for the KMnO4 system, the manganese oxidizing bacteria (MOB; e.g., Pseudoxanthomonas) and metal-resistant genus Acidovorax, dominated the community.

Enhanced membrane fouling control and trace organic compounds removal during microfiltration by coupling coagulation and adsorption in an electric field

Coupling electric field, coagulation, and powdered activated carbon (PAC) adsorption in the microfiltration (MF) process was an effective strategy for membrane fouling alleviation and trace organic compounds (TrOCs) elimination. In the electric field, the surface charges of bovine serum albumin (BSA) molecules and kaolin particles distributed along the direction of the electric field and formed electric dipoles, which lowered electrostatic repulsion between BSA-BSA, BSA-kaolin, and kaolin-kaolin, resulting in enhanced particle aggregation and turbidity reduction. Electrophoretic migration also strengthened the interaction between particles and polyaluminum chloride (PACl). Protein removal showed a significant linear correlation with the transmembrane pressure (TMP), and the slope variation of the fitting curves reflected the role of the electric field in the BSA removal and membrane fouling control. Under the synergistic function of electric field, coagulation, and PAC adsorption, the MF system achieved an 80.7% reduction on membrane fouling, an average BSA removal of 76.4%, and TrOCs elimination of 65.3%-81.7%. Electrochemical oxidation was the main contributor to the TrOCs removal when 2.5 V voltage was applied, and could also prolong the service life of PAC.

Emerging and advanced membrane technology for wastewater treatment: A review

Over the years, conventional wastewater treatment processes have achieved to some extent in treating effluents for discharge pints. Development in wastewater treatment processes is essential to make treated wastewater reusable for industrial, agricultural, and domestic purposes. Membrane technology has emerged as an ideal technology for treating wastewater from different wastewater streams. Membrane technology is one of the most up-to-date advancements discovered to be successful in fundamentally lessening impurities to desired levels. In spite of having certain impediments, membrane bioreactors (MBRs) for biological wastewater treatment provide many advantages over conventional treatment. This review article covers all the aspects of membrane technology that are widely used in wastewater treatment process such as the principle of membrane technology, the classification of membrane technology processes in accordance to pressure, concentration, electrical and thermal-driven processes, its application in different industries, advantages, disadvantages and the future prospective.

Beneficial impacts of natural biopolymers during surface water purification by membrane nanofiltration

Membrane filtration in various forms has become an increasingly used treatment method worldwide for the supply of safe drinking water. The fouling of membranes is commonly considered to be the major operational limitation to its wider application since it leads to frequent backwashing and a shortening of membrane life, and increased production costs. The components of natural organic matter (NOM) in surface waters have been reported previously to be important foulants of nanofiltration (NF) membranes, however, the potential beneficial effect of particular components of these 'foulants' has not been investigated or demonstrated to date. In this study, we have considered the roles of different organic materials including autochthonous NOM (e.g., biopolymers) and allochthonous NOM (e.g., humic substances) on the fouling of NF membranes by bench-scale tests with samples of two representative source waters (UK) taken in two different seasons (autumn and winter). Microfiltration (MF) and ultrafiltration (UF) were employed to generate two permeates, between which the presence of biopolymers (30 kDa - 90 kDa) is the major difference. We developed sequential filtration (MF/UF-NF) to investigate biopolymers' behaviours in NF process. The results showed that the accumulation of biopolymers on NF membranes can mitigate fouling by providing a protective layer in which medium-low molecular weight (MW) materials (e.g. humic substances) are separated by adsorption and/or size exclusion. The protective layers assisted by biopolymers were seen to be thicker under scanning electron microscope (SEM) observation and characterized by higher roughness (i.e. three-dimensional, spacial structure) and greater adsorptive capacity. Moreover, improvement on NF membrane fouling mitigation could be more significant in autumn, comparing to that in winter. The findings in this study were found to be repeatable in similar tests with samples of comparable raw waters in China, and will be important to the practical application of NF membrane systems in terms of a new approach to combating fouling in long-term operation.

Precoating membranes with submicron super-fine powdered activated carbon after coagulation prevents transmembrane pressure rise: Straining and high adsorption capacity effects

Commercially available powdered activated carbon (PAC) with a median diameter of 12-42 μm was ground into 1 μm sized superfine PAC (SPAC) and 200 nm sized submicron SPAC (SSPAC) and investigated as a pretreatment material for the prevention of hydraulically irreversible membrane fouling during a submerged microfiltration (MF) process. Compared with PAC and SPAC, SSPAC has a high capacity for selective biopolymer adsorption, which is a characteristic found in natural organic matter and is commonly considered to be a major contributor to membrane fouling. Precoating the membrane surface with SSPAC during batch filtration further removes the biopolymers by straining them out. In lab-scale membrane filtration experiments, an increase in the transmembrane pressure (TMP) was almost completely prevented through a precoating with SSPAC based on its pulse dose after coagulation pretreatment. The precoated SSPAC formed a dense layer on the membrane preventing biopolymers from attaching to the membrane. Coagulation pretreatment enabled the precoated activated carbon to be rinsed off during hydraulic backwashing. The functionality of the membrane was thereby retained for a long-term operation. Precoating the membranes with SSPAC after coagulation is a promising way to control membrane fouling, and efficiently prevents an increase in the TMP because of the straining effect of the SSPAC and the high capacity of the SSPAC to adsorb any existing biopolymers.

Membrane fouling potential of the denitrification filter effluent and the control mechanism by ozonation in the process of wastewater reclamation

A process of denitrification filter (DNF) coupled with ultrafiltration (UF) and ozonation (DNF-UF-O₃) has been widely applied to advanced nitrogen removal for wastewater reclamation. Despite of the effective removal of nitrogen by DNF, the influence of DNF stage on the operation of UF was still unclear. In this study, a laboratory filtration system was used to investigate the membrane fouling potential of DNF effluent and the fouling control of ozonation. The membrane fouling potential was proved to be increased significantly after DNF stage and alleviated with ozonation treatment. With the help of UV-vis, fluorescence spectroscopy, scanning electron microscopy (SEM) and molecular weight (MW) analysis, the change of DOM component characteristics was proved to be in accordance with the change of fouling potential. The water samples were further fractionated into six hydrophobic/hydrophilic acidic/basic/neutral fractions, among which hydrophobic acids (HOA) and hydrophobic neutrals (HON) dominated the membrane fouling potential of DNF effluent. Detailed study of each fraction revealed that higher MW components in HOA and HON played a crucial role in the fouling of UF membrane. The dominant component of membrane fouling could be degraded and removed by ozonation, and therefore significant fouling alleviation was achieved. These results indicated that in the process of wastewater reclamation, besides conventional water quality indexes, more detailed water features should also be taken into consideration to optimize the whole process. Moreover, the control effects by ozonation could be monitored simply according to the change of specific UV absorbance (SUVA) and fluorescence intensity as surrogates in engineering applications. According to these results, a modified DNF-O₃-UF process with O₃ dosage of 3 mg/L was proposed simply by reversing the sequence of UF and O₃ with no more infrastructure. This modified DNF-O₃-UF process was expected to enlarge the produce capacity of reclaimed water with much lower electricity costs and chemical consumption.

Analyzing and verifying the association of spiral-wound reverse osmosis membrane fouling with different secondary effluents: full-scale experiments

In order to analyze and verify the association of the reverse osmosis (RO) membrane fouling with water quality in full-scale plants, two RO systems (40, 000 m³/d and 20, 000 m³/d) treating different secondary effluents were operated in parallel. The quality of secondary effluents and the performance of RO systems were monitored over 12 months. Difference in foulants distribution and fouling layer composition between the two systems were evaluated by membrane autopsy and foulants characterization. Results verified that: 1) the secondary effluent from municipal sewage caused more serious membrane fouling; 2) more foulants deposited on the surface of leading membrane both in two systems (3.11 ± 0.15 g/m² and 2.93 ± 0.13 g/m²); 3) the microbial community on the RO membrane surface contained more colonizing bacteria in the system treating municipal sewage secondary effluent ; 4) organics in the secondary effluent facilitated biofouling while higher ion concentration restrained biofouling.

The Fe-N-C oxidase-like nanozyme used for catalytic oxidation of NOM in surface water

The removal of natural organic matter (NOM), particularly humic substances (HS) from surface waters during drinking water treatment is necessary to avoid various water quality problems in supply, such as the formation of disinfection by-products. As an alternative to conventional processes (e.g. coagulation), and in the light of the rapidly increasing applications of nanozyme in bio-catalysis, a novel Fe-N-C oxidase-like nanozyme (FeNZ) has been prepared and used to catalyze the oxidative degradation of NOM during simple aeration. Using humic acid (HA) as a model NOM it was found that the HA removal (as TOC) was increased by a factor of 6 with a low dose (10 mg/L) of FeNZ compared to an aerated solution without FeNZ. A variety of analytical methods was used to investigate the oxygen reduction reaction, including cyclic voltammetry, electron spin resonance, and density functional theory (DFT) simulation. Based on these studies, a catalytic oxidation mechanism described as "adsorption-activation-oxidation" was proposed. The enhanced NOM removal performance of FeNZ catalytic oxidation was confirmed with samples of natural surface water in terms of organic mineralization and conversion of hydrophobic to hydrophilic components. The results show great potential for the use of oxidase-like nano catalytic materials in the field of water treatment.

An overview of the uses of high performance size exclusion chromatography (HPSEC) in the characterization of natural organic matter (NOM) in potable water, and ion-exchange applications

Natural organic matter (NOM) constitutes the terrestrial and aquatic sources of organic plant like material found in water bodies. As of recently, an ever-increasing amount of effort is being put towards developing better ways of unraveling the heterogeneous nature of NOM. This is important as NOM is responsible for a wide variety of both direct and indirect effects: ranging from aesthetic concerns related to taste and odor, to issues related to disinfection by-product formation and metal mobility. A better understanding of NOM can also provide a better appreciation for treatment design; lending a further understanding of potable water treatment impacts on specific fractions and constituents of NOM. The use of high performance size-exclusion chromatography has shown a growing promise in its various applications for NOM characterization, through the ability to partition ultraviolet absorbing moieties into ill-defined groups of humic acids, hydrolysates of humics, and low molecular weight acids. HPSEC also has the ability of simultaneously measuring absorbance in the UV-visible range (200-350 nm); further providing a spectroscopic fingerprint that is simply unavailable using surrogate measurements of NOM, such as total organic carbon (TOC), ultraviolet absorbance at 254 nm (UV₂₅₄), excitation-emission matrices (EEM), and specific ultraviolet absorbance at 254 nm (SUVA₂₅₄). This review mainly focuses on the use of HPSEC in the characterization of NOM in a potable water setting, with an additional focus on strong-base ion-exchangers specifically targeted for NOM constituents.

Differences in behaviour of three biopolymer constituents in coagulation with polyaluminium chloride: Implications for the optimisation of a coagulation-membrane filtration process

Coagulation is often applied as a pre-treatment for membrane processes to reduce dissolved organic matter and to prevent membrane fouling. Biopolymers (BPs) have repeatedly been reported as major organic foulants, and coagulation conditions such as pH or dose have been optimised to minimise the remaining BPs. Optimisation however remains problematic because of the complex and heterogenetic nature of BP. In this study, the behaviour of several BP fractions in a coagulation process was investigated by excitation-emission matrix-parallel factor analysis (EEM-PARAFAC) following liquid chromatography (LC)-fractionation. Using a series of jar tests, we found that BP removal depends on the type of source water, reflecting differences in charge neutralisation conditions in three samples of natural water despite nearly identical processes for removing humic substances. This result demonstrates the complexity of optimisation for BP coagulation. Fractionation of EEM-PARAFAC to BP by LC showed that at least three organic component groups (C1, C2 and C3) constitute BP. C1 is tryptophan-like organic matter that is often found in wastewater effluent, C2 is tyrosine-like organic matter that has a phenolic chemical structure, and C3 is a humic-like substance. C1 was removed thoroughly at acidic pH but not at neutral pH, while the removal of C2 was inefficient even with a significant change in pH or dose, indicating similar difficulties in a coagulation process. The difference in components C1 and C2 may partly explain the difference in efficiencies of removal of BP in water from different sources. Our investigation suggests that the optimisation or selection of appropriate pre-treatment processes for membrane systems should be substantially based on the composition of BPs (e.g., C1 and C2 components).

Applying ultraviolet/persulfate (UV/PS) pre-oxidation for controlling ultrafiltration membrane fouling by natural organic matter (NOM) in surface water

Membrane fouling is a recognized obstacle for the application of ultrafiltration (UF) for drinking water treatment. In this study, ultraviolet/persulfate (UV/PS) oxidation was employed as a pretreatment to control membrane fouling caused by natural organic matter (NOM) in surface water. The effects of UV/PS pretreatment on amounts and characteristics of NOM were investigated in terms of dissolved organic carbon, fluorescent spectrum, molecular weight distribution and hydrophobicity. UF membrane fouling during filtration of raw and pre-oxidized water was compared with transmembrane pressure development, and the fouled membranes were further characterized using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The results indicate that NOM was considerably degraded and partially mineralized (∼58%) by UV/PS pretreatment at a PS dose not exceeding 0.6 mM and a UV irradiation time within 120 min, which was attributed to the generation of sulfate and hydroxyl radicals. The fluorescent compounds in NOM were almost completely degraded (>98%) by the UV/PS pretreatment at a PS dose of 0.4 mM, except for tyrosine-like proteins (∼80%). Moreover, UV/PS pretreatment decreased the ratio of macromolecular compounds and increased the hydrophilic fractions, resulting in reduced NOM adhesion to the membrane. Hence, irreversible fouling by NOM was significantly retarded (∼75%) by the UV/PS pretreatment due to reduction in NOM, and more importantly by preferential degradation of fluorescent, macromolecular and hydrophobic compounds. Fouling control performance was considerably improved at increased PS doses and extended UV irradiation time.